INTRODUCTION. 17 



factorily to impress upon my reader the reasons which have 

 appeared to my own mind incontrovertible. 



The question is, — are the Desmidiese animals or vege- 

 tables ? 



The arguments 1 have seen advanced in support of their 

 animal nature appear to me so inconclusive, when contrasted 

 with those adduced in favour of their being vegetables, that, 

 although in the course of a long scientific correspondence 

 I have sought to become thoroughly acquainted with the 

 facts relied upon by the advocates of the former opinion, 

 I have at times almost doubted whether my distance from 

 the metropolis may not have precluded me from the oppor- 

 tunity of hearing others of a more convincing description. 



I will however claim the merit of being at least desirous 

 of stating the case impartially, and I have in vain consulted 

 some distinguished naturalists who differ from me, that I 

 might learn whether their opinions were supported by other 

 reasons than those which are so generally known, and to 

 which I shall presently refer. I will also add, what may be 

 a fact of some weight, that I formerly considered the Des- 

 midieae animals, and the Diatomaceae vegetables, and that 

 careful observations alone have in a great measure reversed 

 my opinions. The Desmidiese I now believe have as strong 

 a claim as the Conjugatae or Palmellse can have to rank with 

 the Algce. On the other hand I consider the proper station 

 of the Diatomaceae very doubtful. They have at least as 

 much right to a place in the animal as in the vegetable king- 

 dom ; and perhaps the safest course would be that adopted 

 by several celebrated continental naturalists, who regard 

 them as belonging to a distinct and intermediate group, and 

 partaking almost equally of the characters of animal and of 

 vegetable. 



The chief reasons advanced by Ehrenberg and others for 

 placing the Desmidieae in the animal kingdom are the fol- 

 lowing : — that they exert a voluntary motion ; that they in- 

 crease by transverse self-division ; and that the Closteria 

 have at their extremities apertures and protruding organs 



c 



